1. Field of the Invention
The present invention relates generally to an adhesive vehicle, to an electrically conductive adhesive prepared therefrom, to electronic circuit structures made with the electrically conductive adhesive, and to processes for the preparation thereof.
2. Description of Related Art
Isotropically electrically conductive adhesives (ECAs) have been used in electronic packaging for over 30 years, primarily as silver filled die attach adhesives. In the 1990's, considerable effort was made to evaluate the use of ECAs for surface mount technology (SMT). It was found that impact shock resistance of ECAs was poor. Card flexing or impact drops as little as two feet were sufficient to fracture ECA joints. Work was also completed in the area of flip chip ECA bumping and interconnection to organic substrates. In this case, the already commonly practiced underfilling provided mechanical reinforcement of ECA joints. Flip chip structures were found to withstand repeated six foot drop tests without measurable contact resistance change at the low million level. Silver was the filler of choice in the industry as it provided excellent conduction and its oxide is semiconducting. Non-semiconducting oxides were found to be highly resistive and resulted in higher contact resistance that would also increase with time. ECAs containing nickel or tin or copper provide much higher contact resistances initially with typical increases of 2 to 10 times during environmental stressing. Silver filled ECAs are used in niche applications for electrical grounding or attachment of components in very temperature sensitive applications. An emerging niche application for ECAs is via fill. Organic carriers require high density wiring and stacked via interconnection for full area array flip chips. Cores are made with vertical vias electrically connecting circuit lines and voltage and ground planes. These cores are then stacked and vias are aligned from one core to another. Lamination joins the cores as well as electrically connects the vias. Electrically conductive adhesives that are B-stageable are easily joined during lamination similar to the typically fiber or particle reinforced B-staged resin of the laminate core. The resulting structure is monolithic and, thus, stresses and strains are averaged over large areas and not highly concentrated at interconnect points. ECA filled vias are reinforced and encapsulated by the surrounding laminate structure.
However, there are undesirable attributes of silver filled ECAs. The best electrically performing silver filled ECAs use flakes that result in platelet stacks having multiple contact points. Flakes are not desirable for filling fine via holes that have diameters less than 100 μm and more especially less than 75 μm. During the via hole screening process, the flakes tend to stack up at the entry to the hole and block further ingress of material. Silver has high ion mobility that results in silver migration and shorting between circuits that are biased. Further, silver is a precious metal and thus, expensive.
Accordingly, there remains a need in the art to discover ECAs that overcome the disadvantages of the prior art, and, in particular, are useful for filling fine hole vias, and are less expensive than silver filled ECAs.